Acoustic foam structures are used for soundproofing. The acoustic foam attenuates airborne sound waves by increasing air resistance, thus reducing the amplitude of the waves. The energy is dissipated as heat.
An acoustic foam structure can be used in, for example, a sound baffle and a noise barrier. A sound baffle primarily absorbs sound, whereas the noise barrier primarily stops sound from travelling from one place to another (i.e. sound insulation).
A sound baffle may comprise an acoustic foam structure that reduces the strength of airborne sound. Sound baffles may be applied to walls and ceilings in building interiors to absorb sound energy and thus lessen reverberation.
A noise barrier may comprise an acoustic foam structure. The noise barrier mitigates noise in places such as roadways, railways and industrial sites.
Open-celled materials and fibre-based materials are known to absorb sound via a porous absorption mechanism. Closed-cell products such as foam have also been shown to absorb sound, but through a membrane mechanism rather than the porous absorption mechanism. An acoustic foam material may be perforated to exploit both the porous absorption mechanism and the membrane mechanism.
U.S. Pat. No. 5,962,107 discloses a low density foam material that is perforated to form a perforated cellular sound absorption material. U.S. Pat. No. 5,962,107 discloses samples that were drilled to a 50% penetration depth and a 100% penetration depth. The samples had stepped perforations.
For example, EP 1 115 777 B1 discloses that the usefulness of a particular polymeric foam in sound management (for example, sound absorption and sound insulation) is partly dependent upon the foam having a substantially open-cell structure. The document further discloses that depending on the percentage of closed-cells that are to be opened, the application of the means for opening can extend only partially through the thickness of the base foam.
WO 2010/036562 A1 discloses laminating two or more perforated polymeric foams together such that a perforated surface of one perforated foam adheres to a perforated surface of another perforated polymeric foam to form an acoustical polymeric foam. Each perforated polymeric foam has perforation channels that extend in a continuous linear fashion all the way through the perforated polymeric foam.
An object of the present invention is to provide a method of perforating a foam structure. Another object is to provide an acoustic foam structure.